Microwave tube cathode of the bariumcalcium aluminate type



April 29,1969 COOPER 3,441,781

MICROWAVE TUBE CATHODE OF' THE BARIUM-CALCIUM ALUMINATE TYPE Original Filed April 5. 1965 INVENTOQ gmv' mk 6% a:

United States Patent US. Cl. 313-346 2 Claims ABSTRACT OF THE DISCLOSURE A microwave tube of the barium-calcium aluminate type comprises a stack of members of a metal selected from the group consisting of tungsten and molybdenum, assembled to leave narrow spaces between parts of adjacent members in the stack, and a filling comprising at least mainly aluminate in the spaces, the working surface of the cathode comprising areas which are at least mainly of aluminate separated by areas of the selected metal. The members may be annular with central holes fitted over a central support member, and may be separated by interspersed spacers of smaller diameter than the members so as to form gaps between the outer portions of the members which are filled with the aluminate material which may comprise a mixture of 40% by weight of barium-calcium aluminate mixture and 60% by weigh-t of molybdenum or tungsten powder.

This is a continuation of Ser. No. 445,348 filed Apr. 5, 1965, now abandoned.

This invention relates to cathodes for magnetrons. As is known, and by definition, a magnetron is essentially a diode vacuum tube in which the fiow of electrons is controlled by an externally applied magnetic field to generate power at microwave frequencies.

A requirement for a cathode to be satisfactory for use in a magnetron, especially in a magnetron which is to be employed as a pulsed magnetron designed to be run with relatively low applied anode voltage and high current, is that it shall have a reasonably high secondary emission coefiicient. This is, of course, because, in operation, as is well known, a large percentage of the current drawn by the magnetron is supplied by secondary electrons.

One of the most satisfactory types of cathode yet produced for pulsed magnetrons is the so-called impregnated barium-calcium aluminate cathode. The usual known form of this cathode consists of a dense sintered, porous tungsten matrix which is impregnated with bariumcalcium aluminate by bringing it into contact with this material in the molten state. This type of cathode has the advantages, over the earlier conventional nickel matrix oxide type of cathode, of longer operational life, and capability of use at longer pulse lengths with improved performance as respects sparking. It has, however, as at present known, the defectsagain as compared with the earlier conventional nickel matrix oxide type of cathode -of being considerably more expensive to make and having a considerably lower secondary emission coeflicient. The cost of making a known impregnated bariumcalcium aluminate cathode is high because, in order to satisfy the close mechanical tolerances usually called for, it is necessary to perform machining operations when the tungsten matrix is filled with copper and then to remove the copper and replace it with the barium-calcium aluminate, The main reason Why known impregnated barium-calcium aluminate cathodes have relatively low ice secondary emission coeflicients, is that, for reasons of mechanical strength, it is necessary to use a dense tungsten matrix, with the result that (to quote a typical figure from present-day practice), about of the final cathode surface is tungsten and only about 20% is aluminate and, of course, the secondary emission coeificient of tungsten is considerably below that of the aluminate mixture. Indeed the secondary emission coetficient of a known impregnated barium-calcium aluminate cathode is so low as, practically speaking, to preclude its use in some pulsed magnetrons, especially some of those designed for operation at low anode voltages. The present invention seeks to provide improved barium-calcium aluminate cathodes in which the above mentioned defects of high cost of manufacture and relatively low secondary emission coefiicient are substantially reduced.

According to this invention, a barium-calcium aluminate cathode comprises a stack of tungsten or molybdenum members assembled to leave narrow spaces between parts of adjacent members in the stack, and a filling consisting at least mainly of aluminate in said spaces so that the working surface of the cathode consists of areas which are at least mainly of aluminate separated by areas of tungsten or molybdenum, as the case may be. The expression mainly of aluminate just employed, and similar statements in this specification indicating that the filling is mainly of aluminate are used to mean that the filling is mainly aluminate by volume. As will be seen later the filling is not necessarily mainly aluminate by weight and, indeed, in the preferred embodiments of the invention it is not so. Preferably the ratio of at least mainly aluminate filling to tungsten or molybdenum in the working surface area of the cathode is approximately unity or more.

A preferred construction of cathode in accordance with the invention comprises a central support member, a stack of annular tungsten or molybdenum members with their central holes fitted over the central support member and alternating along said member with annular spacerrings of the same material so as to leave annular gaps between adjacent members, and a filling consisting at least mainly of barium-calcium aluminate mixture in said gaps.

A preferred method of making a cathode in accordance with this invention comprises the steps of making a plurality of tungsten or molybdenum members and a plurality of space-rs of the same material, forming a stack of said members and spacers with the members and spacers alternating so as to leave thin gaps outwardly of the spacers and between adjacent members, and substantially filling the gaps with a mixture consisting at least mainly of barium-calcium aluminates by bringing the assembled stack into contact with said mixture when in the molten state so that the gaps substantially fill with said mixture as a result of surface tension forces,

Preferably a quantity of tungsten or molybdenum powder is mixed in with the aluminate material. The addition of such powder considerably improves the final cathode not only by increasing the area of contact between aluminate and metal, thus increasing the rate of liberation of free barium but also by militating against the accumulation of surface charges on the surface of the aluminate, which is, of course, an insulating material. The result is a cathode of longer useful life and considerably improved performance as regards sparking. The quantity of tungsten or molybdenum powder mixed in with the aluminate is not critical but a mixture of approximately 60% tungsten or molybdenum powder with 40% aluminate (by weight) gives excellent results. Owing to the much greater specific gravity of tungsten or molybdenum as compared with aluminate, such a mixture (60% metal and 40% aluminate by weight) is still mainly of aluminate by volume.

The invention is illustrated in the accompanying drawing which shows schematically and in section on one side of the centre line thereof, a preferred embodiment.

Referring to the drawing, the cathode structure therein shown comprises a central tube 1 on which is a stack of tungsten or molybdenum annular members 2 which alternate with and are spaced by ring spacers 3 of the same material. The ring spacers 3 and members 2 are centralised by the central tube 1 so that they are concentric with one another and with the tube and, after assembly, the parts comprising the stack are clamped or brazed together. As will be apparent, annular gaps or spaces are left outside the ring spacers 3 between adjacent annular members 2. These gaps or spaces are then substantially filled with a mixture of barium-calcium aluminates with tungsten or molybdenum powder (a suitable mixture is approximately 60% metal and approximately 40% aluminate by weight) by bringing the assembled stack into contact with the mixture in a molten state so that the available volume is substantially filled as a result of surface tension forces, the gaps or spaces being then enough for this to happen. 5 are the usual cathode structure end hats.

The illustrated construction is comparatively inexpensive to make because it dispenses with expensive machining operations necessary in known impregnated bariumcalcium aluminate cathodes. Also a large part of the cost of producing a conventional known barium aluminate cathode is incurred in filling a tungsten or molybdenum matrix with copper before machining it and then removing the copper afterwards. This costly process is unnecessary with a construction in accordance with this invention. Moreover the ratio of aluminate to tungsten (or molybdenum) in the surface area of the cathode-the external cylindrical area-can readily be made 1:1 or higher so that a better secondary emission coeflicient is readily obtainable than with the known impregnated cathodes. Molybdenum is preferred .to tungsten because it gives faster rates of the chemical action necessary to this type of cathode and compensates better for the reduced area of contact (as compared to the known impregnated cathodes) between aluminates and metal.

I claim:

1. A microwave tube cathode of the barium-calcium aluminate type comprising a stack of members formed of a metal selected from the group consisting of tungsten and molybdenum assembled to leave narrow spaces between parts of adjacent members in the stack; and fillings comprising, by volume, at least mainly aluminate in said spaces, said fillings extending substantially to the outer surface of the stack and the peripheries of said fillings being substantially coincident with the peripheries of said members, the working surface of the cathode comprising areas which are at least mainly of aluminate separated by areas of the selected metal,

2. A cathode as claimed in claim 1 and comprising a central support and annular spacer rings of said selected metal fitted over said central support, said members alternating along said central support with said annular spacer rings so as to leave annular gaps between adjacent members, and said fillings comprising, by volume, at least mainly barium-calcium aluminate mixture in said gaps.

References Cited UNITED STATES PATENTS 1,891,074 12/1932 Winter 313339 3,160,780 12/1964 Coppola 313346 3,201,639 8/1965 Levi 313346 3,210,575 10/1965 Dodolsky 313-346 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

US. Cl. X.R. 

